[Molecular study of ampicillin resistance in clinical isolates of Salmonella]

The purpose of this work was to study the molecular basis of beta-lactamase production in ampicillin-resistant strains of Salmonella spp.

It was performed analytical isoelectric focusing of beta-lactamases produced by a group of 33 strains selected in basis of their resistance phenotype. Plasmid profile analysis and assays of transferable drug resistance were developed. The study was completed by hybridization experiments with an intragenic TEM probe which allowed the location of the bla-TEM gene.

By analytical isoelectrofocusing we found that 26 out of the 27 ampicillin-resistant strains produced beta-lactamases with pl 5.4 and/or 5.6 corresponding to TEM-1 and/or TEM-2 type. Analysis of plasmid DNA revealed in almost all strains plasmids ranging in size from 1.1 to 125 Mdal. This plasmids were responsible of the resistance and, moreover, were able to transfer the resistance by conjugation mechanisms. Southern blot analysis detected the gene that code the TEM beta-lactamase at the 125, 8 and 5.8 Mdal plasmids.

Resistance to ampicillin in the strains of Salmonella studied was due to the presence of TEM type beta-lactamases coded by conjugative plasmids. These plasmids coded also resistance to other antimicrobial agents. Our results showed that the use of a DNA probe to the detect TEM-type beta-lactamases using a non radioactive probe, could be a suitable alternative to isoelectric focusing.